reporter-rhtsupport: fix hint query to use correct URL

We should use rs/problems, not rs/cases/NNNNNNNN/problems.

To make similar bugs easier to spot in the future,
I added error message if we did get an error.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>